I am thinking that for the 17" MBP Retina to come back, the latest Mac Pro, which just came out, should be on the shelves, but also the 4K monitors and at a decent price point. $2000 for a 4K monitor is not enough customers on the desktop.
There is an "eco-system" required to get up to that resolution and be productive. Both Mac and Windows must also be able to use those high resolution images with ease for the images and retina to be worth it.
I think Apple realized that with the 15" MBP retina that resolution needed support from software, desktop monitors, and also on the Windows SW and HW sides to be "productive". At least for the tradional mac professional in graphics and movies. This as opposed to say, just programmers or engineer which don't make up much of the customer base for Windows or Mac.
A 4K image or an image that only looks cool on a Retina display is not going to be that valuable; and hence "productive" to others if they also don't have a retina display or the hardware and software to handle that 4K image seamlessly.